This invention relates generally to Class C common base transistor amplifiers of a solid state transmitter and in particular to an apparatus and method for controlling spurious frequency components of a radio frequency (RF) pulse output from the transmitter.
The suppression of spurious frequency components in cascaded Class C common base transistor amplifiers is described in U.S. Pat. No. 4,928,072, issued May 22, 1990, to John C. Scott and assigned to the present assignee. The circuit described in this patent comprises a capacitor and shunt diode in series between an emitter and ground of each Class C amplifier stage to slow the pulse fall time and thereby eliminate spurious frequencies resulting from parasitic ringing.
The Class C common base amplifier is the most widely used configuration for power stages of radio frequency (RF) signals, particularly from UHF to S band. It is more efficient than other modes of operation (Class A, Class AB, etc.) and provides higher gain and better stability than its common emitter counterpart. An illustrative Class C common base transistor amplifier comprises an NPN transistor having its emitter electrode as an input terminal, its collector electrode as an output terminal, and its base electrode coupled to a reference voltage, typically ground. In this configuration, the collector electrode is biased positively with respect to the emitter electrode.
The operation of this amplifier is described in Scott as follows: during the negative half cycle of the input signal, the base-emitter junction is forward biased. After the negative voltage surpasses the base-emitter voltage drop, the transistor turns on and draws collector current. This current forms the negative half cycle of the output signal. During the positive half cycle of the input signal, the base-emitter junction is reverse biased, and the transistor is turned off. During this time, energy stored in the transistor collector circuit is dissipated in the load to form the positive half cycle of the output signal. Because the transistor is conducting for less than 180 degrees of the RF cycle, this is a very efficient mode of operation. Unfortunately, it is also the mode that produces the worst output spectrum.
During normal operation of a Class C common base amplifier, the current flowing out of the emitter will have a small dc component due to the self bias of the base-emitter junction. This dc component flows through the emitter bias return to ground, whenever the RF signal is present at the transistor input. Because the emitter bias return has a certain amount of inherent inductance, energy will be stored during the RF pulse. At the end of the RF pulse, this energy discharges through the transistor input terminals. This inductive discharge injects holes into the transistor base region, and thereby keeps the transistor on for a period determined by the time constant of the inductance and the input parasitics of the transistor.
Because the transistor is biased on during this transient period, any positive feedback path that exists is allowed to produce spurious oscillations or ringing. These oscillations appear in the output spectrum as spurious frequency components. These frequency components appear as relatively wide peaks in the skirt of the output spectrum profile, as may be seen in FIG. 5. As shown, the amplitudes of many of these components may exceed acceptable levels as typically specified for use in high power radar transmitters. As an example, most high power radar systems in use today require all spurious components to be from 70 to 90 decibels below the carrier frequency amplitude (dBc) in a one kilohertz bandwidth.
A solid state transmitter in high power radar systems uses hundreds or thousands of Class C transistor amplifiers in the output stages, thereby requiring hundreds or thousands of Scott's spurious frequency suppressor circuits. This results in considerable additional electronics affecting size, weight, power, cost and reliability of the radar system.